Generally, in a vehicle, a transmission is provided on an output side of a driving source in order to transmit driving force from an internal combustion engine or an electric motor which is of a driving source, i.e., in order to transmit an output torque to a road surface on the optimum condition according to a vehicle running state. The transmission contains a continuously variable transmission which controls a transmission gear ratio in a non-stage (continuous) manner and a gear drive transmission which controls the transmission gear ratio in a stage (discontinuous) manner. The continuously variable transmission contains a belt type continuously variable transmission including two pulleys, i.e., a primary pulley and a secondary pulley and a belt. The driving force is transmitted from the driving source to the primary pulley. The secondary pulley changes and outputs an output torque transmitted to the primary pulley. The belt transmits the driving force transmitted to the primary pulley to the secondary pulley. The primary pulley and the secondary pulley include two pulley shafts (a primary pulley shaft and a secondary pulley shaft), two movable sheaves (a primary movable sheave and a secondary movable sheave), two fixed sheaves (a primary fixed sheave and a secondary fixed sheave), and a belt sandwiching pressure generating unit. The two pulley shafts are arranged in parallel. The two movable sheaves slide on the pulley shafts in axial directions thereof respectively. The two fixed sheaves face the two movable sheaves in the axial directions thereof respectively, and the fixed sheaves form V-shaped grooves between the fixed sheaves and the movable sheaves respectively. The belt sandwiching pressure generating unit generates belt sandwiching pressure between the movable sheave and the fixed sheave. The belt is entrained in the V-shaped grooves formed in the primary pulley and the secondary pulley.
In the belt type continuously variable transmission, the two movable sheaves slide on the pulley shafts in the axial directions thereof by the belt sandwiching pressure generating unit respectively, which changes widths of the V-shaped grooves formed in the primary pulley and the secondary pulley respectively. Therefore, contact radiuses between the belt and the primary pulley and secondary pulley are changed in a non-stage manner to change the transmission gear ratio in the non-stage manner. That is, the output torque from the driving source is changed in a non-stage manner.
For example, as shown in Japanese Patent Application Laid-Open (JP-A) No. 2001-323978, the belt sandwiching pressure generating means contains one in which the movable sheave is pressed toward the fixed sheave side to generate the belt sandwiching pressure by an oil pressure of a hydraulic chamber. In the belt type continuously variable transmission, sometimes a position in the axial direction of the movable sheave is kept constant with respect to the fixed sheave to fix the transmission gear ratio. In this case, in the conventional belt type continuously variable transmission shown in JP-A No. 2001-323978, it is necessary that the oil pressure of the hydraulic chamber be held at a constant oil pressure in order to keep the belt sandwiching pressure constant.